1. Field of the Invention
The present invention relates to a method of detecting a position and an apparatus therefor which are suitable for use in an auto-focusing mechanism of an exposure apparatus used in a lithography process to manufacture a semiconductor device or a liquid crystal device, or a leveling mechanism.
2. Related Background Art
In a projection type exposure apparatus for transferring a circuit pattern formed on a reticle onto a wafer through a projection optical system, the focal depth of the projection optical system has been reduced to a very small value as the microminiaturization of the circuit pattern progresses. Thus, an auto-focusing (AF) system disclosed in Japanese Laid-Open Patent Application No. 56-42205 or U.S. Pat. No. 4,558,949 is used to detect a deviation of a wafer surface from a preset virtual reference plane (which substantially coincides with a focal plane of the projection optical system) with respect to the optical axis direction of the projection optical system to finely drive the wafer in the optical axis direction of the projection optical system to attain focusing. In this AF system, a slit pattern image is obliquely projected onto a predetermined point in a shot area on the wafer without passing through the projection optical system and a reflection image thereof is photoelectrically detected by a synchronous detection system. This system is usually called a fixed point AF system. Normally, since the circuit pattern formed in the shot area on the wafer includes many linear patterns extending along the X and Y directions, the slit pattern image is inclined on the wafer by 45.degree. with respect to the X and Y directions so that the linear patterns cross the slit pattern image. In this manner, the reduction of the measurement precision of the AF system due to the circuit pattern on the wafer is prevented.
As described above, in the fixed point AF system, the deviation of the focal plane from the wafer surface is not directly detected. Thus, if the virtual reference plane shifts from the focal plane of the projection optical system by a drift or the like, or if the focal plane shifts by a change in focusing characteristics of the projection optical system due to absorption of exposing light, the shift appears as a residual focus offset when the pattern is projected and exposed on the wafer. A method for reducing the residual focus offset is disclosed in U.S. Pat. No. 4,650,983, U.S. Pat. No. 4,629,313, and U.S. Pat. No. 4,952,815. In U.S. Pat. No. 4,650,983, a reference pattern is provided on a stage (holder) on which the wafer is mounted, and the reference pattern is reverse-projected onto a specific pattern on the reticle through the projection optical system. The level of the stage is adjusted such that the contrast of the image of the reference pattern formed on the specific pattern is maximum. Thereafter, the AF system is calibrated such that a plane on which the reference pattern is formed is detected by the AF system as a best focus plane (best focal plane). In U.S. Pat. No. 4,629,313, a slit-shaped photosensor is used as a reference pattern on the stage, and the contrast of a pattern image when a slit pattern on the reticle is projected by the projection optical system is detected to determine the best focal plane. In U.S. Pat. No. 4,952,815, a slit-shaped light-emitting mark is provided on the stage, and the image of the light-emitting mark is reverse-projected onto a specific mark on the reticle. Then, the stage is driven in the X and Y directions and light transmitted above the reticle when the specific mark is scanned by the light-emitting mark is photoelectrically detected to detect the best focal plane.
Since there is a partial step in each shot area on the wafer, it is difficult for the fixed point AF system to make the entire plane of the shot area to coincide with the focal plane of the projection optical system. The projection type exposure apparatus is also provided with a leveling sensor disclosed in U.S. Pat. No. 4,558,949 but it is difficult to exactly detect the gradient of the shot area which includes the step. A method for attaining high degree of focusing over the entire plane of the shot area is disclosed in U.S. Pat. No. 5,118,957, in which a pinhole image is obliquely projected onto each of a plurality of (for example, five) points in the shot area on the wafer without passing through the projection optical system, and reflected images therefrom are collectively sensed by a two-dimensional position detection device (CCD). This method is commonly called as an oblique projection multi-point AF system and it can attain focus detection and gradient detection with a high precision.
An assignee of the present application has also filed U.S. patent application Ser. No. 964,954 (on Oct. 22, 1992) which relates to a plane level detection apparatus for detecting a level (a position along an optical axis) at each of a plurality of points in the shot area in order to attain high degree of focusing over the entire plane of the shot area. In the prior art multi-point AF system, a plurality of measurement points are fixed in the shot area, but in the apparatus of the application filed by the present assignee, the number and positions of the measurement points can be readily changed in accordance with the step structure in the shot area so that the level at any point in the shot area can be detected. The plane level detection apparatus proposed by the present assignee will now be explained with reference to FIG. 5.
FIG. 5 shows a projection type exposure apparatus having the plane level detection apparatus proposed by the present assignee. A projection optical system 5 transfers a circuit pattern of a reticle (not shown) to an exposure surface (a surface of a resist layer) 1a of a wafer 1. A wafer holder (.theta. table) 2 for holding the wafer 1 is mounted on a wafer stage 3 which is two-dimensionally driven by a drive means 4 in a plane perpendicular to an optical axis AX1 of the projection optical system 5. The wafer holder 2 is finely movable along the optical axis of the projection optical system 5 and tiltable in any direction on the wafer stage 3. The drive means 4 drives the wafer holder 2 in accordance with a command from a correction amount calculation means 19 to attain focusing and leveling of the wafer 1.
The detailed arrangement of the oblique projection type plane level detection system will now be explained. Referring to FIG. 5, illumination light from a light source 6 is substantially collimated by a condenser lens 7 and is incident on a reflection type phase grating plate 8, which has a phase grating formed on its grating formation surface 8a by steps which extend parallelly to the plane of the drawing at a pitch Q1. Usually, the surface of the wafer 1 is covered by a photoresist. In order to reduce an affect of a thin film interference, it is desirable that the light source 6 is a white light source (for example, a halogen lamp). However, a light-emitting diode which emits light having a wavelength band which is of low sensitivity to the resist may be used as the light source 6.
A focusing lens 9 and a projection objective lens 10 form a projection optical system, and an optical axis AX2 of the projection optical system crosses the optical axis AX1 of the projection optical system 5 with an angle .theta.. Reflected (including diffracted light) having a mean reflection angle .gamma., from the grating formation surface 8a is focused on the surface 1a to be examined of the wafer 1 by the projection optical system (9 and 10). The construction is such that when the plane 1a coincides with the best focal plane of the projection optical system 5, that is, in an in-focus state, the surface 1a and the grating formation surface 8a meet a shine-proof condition with respect to the projection optical system (9 and 10). Accordingly, in the in-focus state, the image of the grating pattern of the grating formation surface 8a is correctly focused on the entire surface 1a. The projection optical system (9 and 10) is a bi-telecentric optical system and any point on the grating formation surface 8a and a conjugate point on the surface 1a have the same magnification over the entire surface. Accordingly, since the grating pattern of the grating formation surface 8a in FIG. 5 is an equi-interval diffraction grating having a longitudinal direction along a direction perpendicular to the plane of drawing, the image projected onto the surface 1a is also an equi-interval grating pattern having a longitudinal direction along the direction perpendicular to the plane of the drawing.
A light-receiving objective lens 11 and a focusing lens (imaging lens) 12 form a focusing optical system which focuses reflected light from the surface 1a to a dump correction diffraction grating plate 13. An optical axis AX3 of the focusing optical system (the lenses 11 and 12) is symmetric to the optical axis AX2 of the projection optical system (9 and 10) with respect to the optical axis AX1 of the projection optical system. A phase type grating pattern having steps extending parallelly to the plane of drawing of FIG. 5 at a pitch Q2 is also formed on a grating formation surface 13a of the diffraction grating plate 13. The grating pattern has a predetermined blaze angle, as will be described later. The construction is such that when the surface 1a coincides with the focal plane of the projection optical system 5, the surface 1a and the grating formation surface 13a meet a shine-proof condition with respect to the focusing optical system (11 and 12). Accordingly, in the in-focus state, the image of the grating pattern on the surface 1a is correctly refocused on the entire grating formation surface 13a. The focusing optical system (11 and 12) is also a bi-telecentric optical system, and any point on the surface 1a and a conjugate point on the grating formation surface 13a have the same magnification over the entire surface. Accordingly, when the surface 1a coincides with the focal plane of the projection optical system 5, the image projected onto the grating formation surface 13a is also an equi-interval grating pattern having a longitudinal direction along a direction perpendicular to the plane of drawing.
As described above, in FIG. 5, when the surface 1a coincides with the focal plane of the projection optical system 5, the grating formation surface 8a, the surface 1a, and the grating formation surface 13a meet the shine-proof condition and the magnification on each plane is equal over the entire plane.
Light incident on the diffraction grating plate 13 is diffracted by the grating pattern. The pitch of the grating pattern of the diffraction grating plate 13 is determined such that a principal light beam of the reflected light is substantially parallel to a normal direction to the grating formation surface 13a. The reflected light from the grating formation surface 13a is focused on a light-receiving surface 17a of a two-dimensional charge coupled image pickup device (CCD) 17 through a lens 14, a plane mirror 15, and a lens 16. In this manner, the image of the grating pattern formed on the grating formation surface 13a is refocused on the light-receiving surface 17a. A relay optical system formed by the lenses 14 and 16 is also a bi-telecentric system.
The image formed on the light-receiving surface 17a is produced by relaying twice the image of the grating pattern of the reflection type phase grating plate 8 projected onto the surface 1a. In other words, it is a double refocused image of the pattern image projected on the surface 1a. Since the principal light beam of the reflected light from the diffraction grating plate 13 is substantially perpendicular to the grating formation surface 13a, the principal light beam incident on the CCD 17 is also substantially perpendicular to the light-receiving surface 17a. Since the image of the pattern projected onto the surface 1a is double refocused on the light-receiving surface 17a, the grating pattern image on the light-receiving surface 17a shifts laterally when the surface 1a moves along the optical axis of the projection optical system 5. Accordingly, the plane position along the optical axis AX1 on the surface 1a can be measured by measuring the lateral shift.
More specifically, when the surface 1a is planar, a fringe (light-dark pattern) 22 having light portions 20 and dark portions 21 at a predetermined pitch is focused on the light-receiving surface of the CCD 17, as shown in FIG. 6A. An image pickup signal (voltage) which is an average of image pickup signals along the pitch direction of the fringe 22 within a predetermined range is large in an area corresponding to the light portion 20 and small (close to zero) in an area corresponding to the dark portion 21, as shown in FIG. 6B. As shown in FIG. 7A, if there are a recessed portion 24A and a projected portion 24B in an area (broken line) 23 onto which the light-dark pattern is projected on the surface 1a, the phase of the fringe 22 partially changes on the light-receiving surface of the CCD 17. Namely, as shown in FIG. 7B, the phase of the fringe 22 changes in areas 25A and 25B, corresponding to the recessed portion 24A and the projected portion 24B, on the light-receiving surface of the CCD 17. Accordingly, the position of the entire surface 1a along the optical axis of the projection optical system 5 can be detected by detecting the phases of the respective portions of the fringe 22.
Referring back to FIG. 5, the image pickup signal from the CCD 17 is supplied to a detection unit 18. The detection unit 18 processes the image pickup signal (FIG. 7B) to detect a pattern image on the light-receiving surface 17a, and supplies pattern information to the correction amount calculation means 19. The correction amount calculation means 19 determines a deviation between the surface 1a and the focal plane of the projection optical system 5 based on the pattern information, and controls the drive of the wafer holder 2 through the drive means 4 such that the surface 1a, that is, the entire shot area falls within the focal depth of the projection optical system 5.
When a displacement of the surface 1a along the optical axis AX1 is z, the lateral shift y of the fringe (light-dark pattern image) on the light-receiving surface of the CCD 17 is given by the following formula: ##EQU1## where .theta. is the incident angle of the principal light beam of the illumination light to the surface 1a, .beta. is the lateral magnification of the focusing optical system (11 and 12), .beta.' is the magnification along a focal plane of the dump from the surface 1a to the grating formation surface 13a, and .beta." is the lateral magnification of the relaying optical system (14 and 16).
A function of the dump correction diffraction grating plate 13 will now be described. For example, when the lateral magnification .beta. of the focusing optical system (11 and 12) is 0.5 and the incident angle .theta. is 85.degree., an incident angle .alpha. of the principal light beam of the reflected light from the surface 1a with respect to the diffraction grating plate 13 is approximately 80.degree.. With such a large incident angle, the intensity of incident light is considerably reduced if the CCD 17 is directly arranged in place of the diffraction grating plate 13, because the light incident on the photoelectric conversion portion of the CCD is eclipsed by a peripheral read circuit portion, and surface reflection of the photoelectric conversion portion and a window glass of the CCD package is so large that the light is not effectively incident on the light-receiving portion. Thus, an optical element to reduce the incident angle to the CCD is required. In FIG. 5, it is attained by using 1st-order diffracted light from the diffraction grating plate 13.
In FIG. 5, the diffraction grating plate 13 is used as the dump correction optical member. Alternatively, a prism 13' in FIG. 8 may be used. The plane level detection system shown in FIG. 8 will now be explained. The same reference numerals in FIG. 8 denote the same parts as in FIG. 5, and an explanation thereof will be omitted.
In FIG. 8, reflected light from the surface 1a is focused onto an incident surface 13'a of the prism 13' through the light-receiving objective lens 11, a plane mirror 15A, and the focusing lens 12. When the surface 1a coincides with the focal plane of the projection optical system 5, the surface 1a and the incident surface 13a meet the shine-proof condition with respect to the focusing optical system (11 and 12). Accordingly, in the in-focus state, the image of the grating pattern on the surface 1a is correctly refocused on the entire incident surface 13'a. The refractive index of a glass material of the prism 13' is selected such that the principal light beam of the light refracted by the prism 13' is substantially parallel to the normal direction to the incident surface 13'a. The light beam emerging from the prism 13' is focused on the light-receiving surface 17a of the two-dimensional CCD 17 through a plane mirror 15B and the relay lenses 14 and 16. The principal light beam of the light emerging from the prism 13' is substantially normal to the incident surface 13'a or has a small refraction angle. Accordingly, the principal light beam of the light incident on the two-dimensional CCD 17 is also substantially normal to the light-receiving surface 17a or has a small incident angle .rho..
According to the arrangement shown in FIG. 5 or 8, the position detection is attained at arbitrary point in the predetermined range (light-dark pattern area) on the surface 1a. However, circuit patterns of preceding layers have already been formed on the surface 1a, and the image of the circuit pattern may be superimposed on the fringe 22 of FIG. 6A on the light-receiving surface of the CCD 17. Thus, when a peak of the image pickup signal of FIG. 6B is detected as in the prior art, a partial phase change of the fringe 22 cannot be correctly detected, and a detection error occurs.
The fringe (light-dark pattern image) 22 focused on the light-receiving surface of the CCD 17 is a periodic pattern at a pitch P, as shown in FIG. 7B. For this reason, if the lateral shift (phase shift) amount of the fringe 22 in the pitch direction (a D1 direction in FIG. 7B) is represented by .DELTA.y, the detection range of the lateral shift amount .DELTA.y satisfies, e.g., the following relation: EQU -P/2.ltoreq..DELTA.y&lt;P/2
If the position shift amount of the surface 1a in the optical axis direction of the projection optical system 5 when the lateral shift amount .DELTA.y of the fringe 22 satisfies .DELTA.y=P is represented by Z.sub.P, when the lateral shift amount .DELTA.y exceeds .+-.P/2, the detection result (the position shift amount of the surface 1a) includes an error of n.times.Z.sub.P (n is an integer). This means that the plane level detection system shown in FIG. 5 or 8 has a narrow detection range, in the optical axis direction, of the surface 1a. Therefore, when the level of the surface 1a is detected using the plane level detection system shown in FIG. 5 or 8, the surface 1a must be caused to fall within the detection range in advance using another level detection means, e.g., an air micrometer or the like, and the detection system is undesirably complicated very much.
In the above-mentioned plane level detection system, the relative displacement, in the height direction, of the surface 1a can be detected from the lateral shift amount of the light-dark fringe 22 on the light-receiving surface 17a of the CCD 17. In particular, in the projection type exposure apparatus (FIG. 5 or 8), the position shift amount of the surface to be examined (wafer surface) with respect to the best focal plane of the projection optical system 5 must be detected. Thus, in a state wherein a reference surface on a reference member provided to the wafer stage 3 is caused to coincide with the best focal plane of the projection optical system 5, a light-dark pattern is projected onto the reference surface, the position of the pattern image (light-dark fringe) on the light-receiving surface of the CCD 17 is detected, and the position of the light-dark fringe is stored in advance as a reference position. When the surface of the wafer 1 is caused to coincide with the best focal plane of the projection optical system 5, the position of the pattern image focused on the light-receiving surface of the CCD 17 when the light-dark pattern is projected onto the wafer 1 is compared with the above-mentioned reference position to detect the lateral shift amount of the pattern image (light-dark fringe) with respect to the above-mentioned reference position, thus detecting the position shift amount of the wafer surface with respect to the best focal plane of the projection optical system 5.
However, it is very difficult to calibrate the above-mentioned plane level detection system, i.e., to detect the reference position of the light-dark fringe using the reference surface of the reference member. More specifically, since the above-mentioned fixed point or multi-point AF system for projecting a slit pattern (or pinhole) image on the surface to be examined has a very small pattern image projection area on the surface to be examined, the reference surface of the reference member to be used in calibration can be small, and the flatness of the reference surface does not pose any problem. In contrast to this, since the above-mentioned plane level detection system projects the light-dark pattern on the wide area (having an area as large as that of the shot area on the wafer) 23 on the surface 1a, as shown in FIG. 7A, it requires a reference surface having an area as large as that of the area 23. Although the flatness of such a wide reference surface is important, it is difficult to process the reference surface to a perfectly smooth surface free from steps.
Furthermore, when the best focal plane of the projection optical system 5 is not a flat plane but is deformed to a curved plane due to aberrations such as a curvature of field, the reference surface must also be processed to the same curved surface as the best focal plane. However, such a process is further difficult to attain. In addition, the best focal plane of the projection optical system 5 may gradually deform further due to aging (e.g., exposing light absorption), the above-mentioned reference surface cannot cope with such aging. Therefore, the above-mentioned calibration method of the plane level detection system cannot correctly detect the reference position of the light-dark fringe, i.e., cannot sufficiently reduce a residual focus offset.